Thank you for your question. Your Japanese maple has a bagworm on it. While bagworms are more commonly seen on evergreen trees and can cause serious problems on them, it is not unusual to find a bagworm on other plants. It is typically not a serious pest of deciduous plants. Simply pick it off of the branch and dispose of it in the trash. It would be best to squish it to make sure anything inside is dead. Putting bagworms in the trash without doing this will likely result in it climbing out and attaching somewhere else.
Here is a link to the bagworm information on our website. https://extension.umd.edu/hgic/topics/bagworms-trees-and-shrubs